1. Field of the Invention
This invention relates to systems for analyzing fluids, and more particularly to an apparatus for determining partial pressures of blood gasses, concentrations of electrolytes, and hematocrit value of a fluid sample, and to a method for fabricating such an apparatus.
2. Description of Related Art
In a variety of instances it is desirable to measure the partial pressure of blood gasses in a whole blood sample, concentrations of electrolytes in the blood sample, and the hematocrit value of the blood sample. For example, measuring pCO.sub.2, pO.sub.2, pH, Na.sup.+, K.sup.+, Ca.sup.2+ and hematocrit value are primary clinical indications in assessing the condition of a medical patient. A number of different devices currently exist for making such measurements. Such devices are preferably very accurate in order to provide the most meaningful diagnostic information. In addition, in an attempt to use as little of the patient's blood as possible in each analysis performed, the devices which are employed to analyze a blood sample are preferably relatively small. Performing blood analysis using a small blood sample is important when a relatively large number of samples must be taken in a relatively short amount of time or if the volume of blood is limited, as in neonates. For example, patients in intensive care require a sampling frequency of 15-20 per day for blood gas and clinical chemistry measurements, leading to a potentially large loss of blood during patient assessment. In addition, by reducing the size of the analyzer sufficiently to make the unit portable, analysis can be performed at the point of care. Also, reduced size typically means reduced turnaround time. Furthermore, in order to limit the number of tests which must be performed it is desirable to gather as much information as possible upon completion of each test. However, size limitations are imposed upon the sensors that are used to measure blood chemistry. These size limitations are in large part due to physical geometries of the sensors and the connections to the sensors. In a blood analyzer, disclosed in U.S. Pat. No. 4,818,361 a sensor assembly is fabricated in an attempt to reduce the size of the blood analyzer.
The sensor assembly has a plurality of sensors formed on a front side of a polymeric form along a flow path between an inlet and outlet port. The flow path is formed as a groove in a polymeric form. The form is either molded or machined. FIG. 1 illustrates a cross-sectional view of the sensor assembly. Electrodes are formed and communicate with a measurement flow channel 34 which is formed and which communicates with a measurement flow channel 34 which is established by the combination of substrate 30 and cover plate 32.
FIG. 1 illustrates a pH sensor 10 and a CO.sub.2 sensor 20. Each sensor 10, 20 includes a wire 17 which is inserted through and substantially fills a hole 12 in the form 30. Upon inserting the wire 17 through the hole 12, it is critical to ensure that the wire 17 completely fills the hole 12. Gaps or cavities which form between the inner walls of the hole 12 and the wire 17 act as reservoirs in which contaminants which can contaminate the sensor electrode can be held. An adhesive is used to retain the wire 17 within the hole 12. Use of such adhesive further increases the risk that the electrode will be contaminated. The wire 17 may be friction fit within the hole 12. However, insertion of the wire 17 into a tight fitting hole is very difficult and requires excessive labor. Furthermore, even with the use of an adhesive, there is a risk that due to differences between the coefficient of expansion of the wire 17 and the form 30, the wire 17 will delaminate from the walls of the hole 12 under varying conditions of temperature and humidity
It will be clear that such expansion and contraction can act as a pump, drawing traces of the analyte and/or other contaminates into the cavities between the wire 17 and the walls of the hole 12.
The end of the wire 17 opposite the measurement flow chamber 34 is coupled to electrical conductor 19 which serves as the electrical connection the to pH sensor 10. Conductor 19 may be in the form of a printed circuit conductor which lies upon the surface of form 30.
At the other end of electrode wire 17 is an electrochemically active layer 15. This electrochemically active layer 15 has essentially the same cross-sectional dimension as the wire 17 and serves to electrochemically couple wire 17 to an electrolyte layer 13. Another layer 11 is exposed to the fluid in the measurement channel 34 and covers the electrolyte layer 13. Accordingly, the shape and dimensions of the wire 17 dictate the dimensions of the electrochemically active layer of the sensor. Thus, the sensor dimensions are limited by the dimensions of wire stock which is available. In addition, the shape of the electro-chemically active layer of the sensor is limited by the shape of a cross-section of the wire 17 (i.e., essentially limiting the electro-chemically active layer of the sensor to a circular geometry). Still further, the use of wire 17 to fill the hole 17 places limitations on the thickness of the form, since insertion of the wire 17 into the hole 12 becomes difficult if the wire is short.
Furthermore, since the interface between the electrically active layer is relatively large, the wire must be of a material that is compatible with the electrically active layer to prevent negatively affecting the operation of the sensor. That is, over time, the conductive material of the wire contaminates the material used to form the electro-chemically active layer of the sensor, disrupting the electro-chemical characteristics of the sensor. Therefore, constraints are placed on the material of the wires used to fill the holes 12. In one such assembly, the active layer 15 is formed from siver chloride, and the wire is formed from silver. The remaining portions of the pH sensor 10 are formed in a shallow well 14 which is concentric about the electrode hole 12. The inner layer 13 is an electrolyte layer. The CO.sub.2 sensor 20 is similarly constructed.
In addition to the problems noted above, several other problems exist with this type of sensor. First, the process that is used to fabricate the assembly requires that each sensor assembly be handcrafted. Accordingly, fabrication of the sensor assembly requires a substantial amount oflabor which is expensive and time consuming. The hole 12 associated with each sensor within the assembly must be filled with wire 12 by hand one sensor at a time. In addition to the amount of labor required to fill each sensor hole 12, variations in the quality of the operation and the conditions under which each hole 12 is filled increase the possibility that the entire assembly will operate below an acceptable performance standard due to one of the sensors exhibiting poor performance.
Second, the form 111 is fabricated from a polymeric material that tends to absorb some of the fluid which flows through the flow path 103. This absorbed fluid has a relatively low resistance compared with the very high resistance required between electrodes of the sensors 101. Accordingly, the initially high resistance which exists between the electrodes degrades. As the resistance between the conductive material 301 of each sensor degrades, the accuracy of the sensors 101 degrades as well.
Third, the electrical interface between the assembly and electronics external to the assembly is through an plurality of contacts which are fabricated on the rear surface of the form. These contacts slide against a spring loaded mating contact in the blood analyzer. As the contacts of the sensor assembly slide against the mating contacts within the blood analyzer, the contacts of the blood analyzer are worn down. Therefore, after being inserted and removed from the blood analyzer a number of times, the electrical connection between the external circuits within the blood analyzer and the sensors within the sensor assembly will be degraded.
In addition to these problems, the blood to be analyzed must be heated and regulated to a known stable temperature. Heating and stabilizing the temperature of the blood can take a substantial amount of time. Still further, in many cases analysis must be performed at regular and closely spaced intervals. Accordingly, if the heating and temperature stabilization time is relatively long, the number of times such analysis can be performed within a particular amount of time (i.e., turn around time) can be limited to a number less than would otherwise be desirable.
Accordingly, it would be desirable to provide a sensor which remains accurate over a relatively long period of exposure to electrolytes and blood samples, uses a very small sample size, detects the concentration of a number of different electrolytes and the partial pressure of a number of blood gases all in a single analysis, and in which a blood sample may be heated very rapidly to a known stable temperature.